Phase noise or timing jitter are the ultimate indicators of the quality of oscillators. The quality of an oscillator, whether harmonic or pulsed, is mostly determined by either of these two parameters. There are well-known techniques for measuring the quality of electrical/microwave oscillators. These methods can, in principle, be applied to optical oscillators, as well. Utilization of electrical and/or microwave oscillator techniques for characterizing the quality of optical oscillators, however, involves generating an electrical signal from its original optical counterpart.
The most common and simplest way of converting from the optical domain to the microwave domain is to apply direct photodetection by shining the optical signal on a photoreceiver. It has been shown, however, that during the direct photodetection process, an excess amount of phase noise can be generated. The cause of this excess phase noise is the power fluctuations of the optical signal that are converted to phase fluctuations through a process called amplitude modulation to phase modulation (AM-PM) conversion.
Balanced optical cross correlation, however, has been shown not only to resolve the AM-PM conversion issue to the first order, but also to effectively increase the sensitivity of the jitter measurement by more than three orders of magnitude.
In U.S. Pat. No. 7,940,390 B2, material birefringence is utilized to make an optical cross correlator. In embodiments described therein, a type-II nonlinear crystal is used for cross correlation where the wavelengths are identical or very close (where wavelength filtering at the output is very difficult or impossible). The nonlinear crystal is formed of a material that is transparent to both fundamental and harmonic frequencies, and the material properties of the nonlinear crystal determines the limit as to what pulsewidth can be used. Furthermore, due to the difference in the material platforms for the cross correlator and the photoreceiver, their integration may be difficult; and, even in the case of integration, the integration may be hybrid and not monolithic.
Two photon absorption in semiconductors have been used in the past to perform autocorrelation function for the purpose of pulse duration measurements [F. Laughton, et al., “Very Sensitive Two-Photon Absorption GaAs/AlGaAs Waveguide Detector for an Autocorrelator”, Electronics Letters, Vol. 28, No. 17, 1663-65 (1992); F. Laughton, et al., “The Two-Photon Absorption Semiconductor Waveguide Autocorrelator”, IEEE J. of Quantum Electronics, Vol. 30, No. 3, 838-845 (1994); and U.S. Pat. No. 6,956,652 B2 (Whitbread, et al.)]. Modal phase matching in semiconductor guided wave devices have been used to perform second harmonic generation, sum frequency generation, or difference frequency generation [K. Moutzoris, et al., “Second-harmonic generation through optimized modal phase matching in semiconductor waveguides”, Applied Physics Letters, Vol. 83, No. 4, 620-22 (2003); X. Yu, et al., “Efficient continuous wave second harmonic generation pumped at 1.55 um in quasi-phase-matched AlGaAs waveguides”, Optics Express, Vol. 13, No. 26, 10742-48 (2005); P. Abolghasem, et al., “Type-0 second order nonlinear interaction in monolithic waveguides of isotropic semiconductors”, Optics Express, Vol. 18, No. 12, 12681-89 (2010)]. Here, we exploit the fundamental properties of semiconductors and guided wave devices to invent a unique background-free balanced optical cross-correlator.